Photoelectric and electrophotographic pigments comprising derivatives of condensed polycyclic aromatic hydrocarbon aldehydes

ABSTRACT

An electrophoretic imaging system which is capable of polychromatic or monochromatic imaging is disclosed, using as at least a portion of the imaging material compounds which are the reaction product of a condensed polycyclic aromatic hydrocarbon aldehyde and certain ketones or amines containing active hydrogen. These compounds may also be used in the preparation of electrophotographic plates for xerography.

Unlted States Patent 1191 1111,; 3,923,506 Bergfjord et al. Dec. 2, 1975 I 1 PHOTOELECTRIC AND 3,246,983 4/1966 Sus et a1. 96/].5 ELECTROPHOTOGRAPHIC PIGMENTS 3,414,650 12/1968 Catine et a1 260/566 B 3,510,419 5/1970 Cerrena et a1. 96/1 PE COMPRISING DERIVATIVES 0F 3,546,085 12/1970 Weinberger et al. 96/1.3 CONDENSED POLYCYCLIC AROMATIC HYDROCARBON ALDEHYDES FOREIGN PATENTS OIR APPLICATIONS [75] Inventors; John Alf Bergfjord, Ma don; 930,988 7/1963 United Kingdom 96/].5 Steven James Grammatica Webster; 980,51 1 1/1965 United Kingdom 96/I.5 R h d W'll' 1c ar 1 mm Radler, Manon, all OTHER PUBLICATIONS of NY.

[73] Assign/3ez Xerox Corporation Stamford Weizmann et al., Jour. of Am. Chem. Soc., Vol. 70

Conn (1948), pp 2829-2830.

[221 Filed: Mar. 5, 1973 Primary ExaminerCharles L. Bowers, Jr. I I PP NOJ ,787 Assistant Examiner-J. P. Brammer [44] Published under the Trial Voluntary Protest Program on January 28, 1975 as document no. B 337,787 [57] ABSTRACT An electrophoretic imaging system which is capable of U-S. po]ychromatic o monochromatic is disclosed 204/18 260/566 B using as at least a portion of the imaging material Cl-2G03G (5036 6 compounds which are the reaction product of a con- Fleld of Search PE, densed polycy lic aromatic hydrocarbon aldehyde and 260/566 B; 204/18 PE certain ketones or amines containing active hydrogen. These compounds may also be used in the preparation [56] References Clted of electrophotographic plates for xerography.

UNITED STATES PATENTS I 2,851,495 9/1958 Jensch et a1 260/566 B 7 1 Drawmg F'gme I PHOTOELECTRIC AND ELECTROPHOTOGRAPI'IIC PIGMENTS COMPRISING DERIVATIVES OF CONDENSED POLYCYCLIC AROMATIC HYDROCARIBON ALDEI-IYDES BACKGROUND OF THE INVENTION There has been recently developed an electrophoretic imaging system capable of producing color images which utilizes single-component photoconductive particles. This process is described in detail and claimed in US. Pat. Nos. 3,384,565, 3,384,566 and 3,384,488. In such an imaging system, variously colored light absorbing particles are suspended in a non-conductive liquid carrier. The suspension is placed between electrodes, subjected to a potential difference and exposed to an image. As these steps are completed, selective particle migration takes place in image configuration, providing a visible image at one or both of the electrodes. An essential component of the system is the suspended particles which must be electrically photosensitive and which apparently undergo a net change in charge polarity upon exposure to activating electromagnetic radiation through interaction with one of the electrodes. In a monochromatic system, particles of a single color are used, producing a single colored image equivalent to conventional black'and-white photography. In a polychromatic system, the images are produced in natural color because mixtures of particles of two or more different colors which are each sensitive to light of a specific wavelength or narrow range of wavelengths are used. Particles used in this system must have both intense pure colors and be highly photosensitive. The pigments of the prior an often lack the purity and brilliance of color, the high degree of photosensitivity, and the preferred correlation between the peak spectral response and peak photosensitivity necessary for use in such a system.

Several recent patents have issued dealing with the preparation and use of certain yellow pigments having desirable photoelectrophoretic properties. For example, US. Pat. No. 3,447,922 discloses pigments comprising N-substituted-8,13-dioxodinaptho-(2,l-b; 2 ,3 d)-furan-6-carboxamides. Another class of suitable pigments are l,3,6,8-substituted pyrenes as disclosed in U.S. Pat. No. 3,546,085. While these pigments are suitable for use in photoelectrophoretic imaging processes, the varieties of shades or tones of color produced are limited due to chemical structure of the various compounds. It is always desirable to have available pigments which offer a broader selection of yellow or orange-yellow shades and tones.

The reaction produce of pyrene-3-aldehyde and methyl ketones have been reported in the prior art as disclosed by M. Weizmann and E. Bograchov. Journal of the American Chemical Society, Vol. 70, 2829 (I948). However, the fact that these and related compounds are extremely useful in photoelectrophoretic or xcrographic imaging processes in the form of finely divided pigments has been unrecognized until the present invention.

It is accordingly an object of this invention to provide novel electrophoretic imaging systems.

It is an additional object of this invention to provide a novel imaging suspension comprising an insulating carrier liquid having dispersed therein a finely divided SUMMARY OF THE INVENTION The foregoing objects and others are accomplished by utilizing as a photosensitive component in electrophoretic imaging processes and xerographic processes one or more compounds having the general formulae:

represents a condensed polycyclic aromatic group having from about 14 to about 20 carbon atoms,

R is selected from the group consisting of: H and s si R is selected from the group consisting of: CH;,, C H lower alkyl containing 3-8 carbon atoms,

R is selected from the group consisting of: H. OH, CH OCH C H OC H COCH CO CH COC H CO C H N0 CN, SO NH SO NHC H, Cl, F. Br, I; and

n is a positive integer from 1-3.

These compounds may be prepared by reacting a monosubstituted condensed polycyclic aromatic aldehyde with an amine or ketone containing active hydro gen, as will be hereinafter disclosed. The preferred aldehydes are the monosubstituted carboxaldehydes of pyrene, anthracene, phenanthrene, chrysene, tetracene and tetraphene. The reaction products are formed as the result of an aldol type condensation involving the aromatic aldehyde and a suitable ketone, or a condensation reaction involving the aromatic aldehyde and a primary aliphatic or aromatic amine.

The compounds produced by the above reactions havethe common characteristic of a brilliant, intense yellow or orange color and are readily dispersible in common solvents. They are surprisingly effective when used in monochromatic or polychromatic electrophoretic imaging systems, as well as in preparing electrophotographic plates as will be hereinafter defined.

Their good spectral response and high photosensitivity result in dense, brilliant photoelectrophoretic images.

DETAILED DESCRIPTION OF THE INVENTION The use of the above compounds in electrophoretic imaging will be better understood upon reference to the drawing which shows schematically an exemplary electrophoretic imaging system.

Referring now to the FIGURE, there is seen a transparent electrode generally designated 1 which, in this exemplary instance, is made up of a layer of optically transparent glass 2 overcoated with a thin optically transparent layer 3 of tin oxide, commercially available under the name NESA glass. This electrode will hereafter be referred to as the injecting electrode. Coated on the surface of injecting electrode 1 is a thin layer 4 of finely divided photosensitive particles dispersed in an insulating liquid carrier. The term photosensitive for the purposes of this application, refers to the properties of a particles which, once attracted to the injecting electrode, will migrate away from it under the influence of an applied electric field when it is exposed to actinic electromagnetic radiation. For a detailed theoretical explanation of the apparent mechanism of operation of the invention, see the above-mentioned US. Pat. Nos. 3,384,565, 3,384,566 and 3,384,488, the disclosures of which are incorporated herein by reference. Liquid suspension 4 may also contain a sensitizer and- /or a binder for the pigment particles which is at least partially soluble in the suspending or carrier liquid as will be explained in greater detail below. Adjacent to the liquid suspension 4 is a second electrode 5, hereinafter called the fblocking electrode" which is connected to one side of the potential source 6 through a switch 7. The opposite side of potential source 6 is connected to the injecting electrode 1 so that when switch 7 is closed, an electric field is applied across the liquid suspension 4 between electrodes 1 and 5. -An image projector made up of a light source 8, a transparency 9, and a lens 10 is provided to expose the dispersion 4 to a light image of the original transparency 9 to be reproduced. Electrode 5 is made in the form of a roller having a conductive central core 11 connected to the potential source 6. The core is covered with a layer of a blocking electrode material 12, which may be Baryta paper. The pigment suspension is exposed to the image to be reproduced while a potential is applied across the blocking and injecting electrodes by closing switch 7. Roller 5 is caused to roll across the top surface of injecting electrode 1 with switch 7 closed during the period of image exposure. This light exposure causes exposed pigment particles originally attracted to electrode l to migrate through the liquid and adhere to the surface of the blocking electrode, leaving behind a pigment image on the injecting electrode surface which is a duplicate of the original transparency 9. After exposure, the relatively volatile carrier liquid evaporates off, leaving behind the pigment image. This pigment image may then be fixed in place as, for example, by placing a lamination over its top surface or by virtue of a dissolved binder material in the carrier liquid such as paraffin wax or other suitable binder that comes out of solution as the carrier liquid evaporates. About 3 to 6 percent by weight of paraffin binder in the carrier has been found to produce good results. The carrier liquid itself may be liquefied paraffin wax or other suitable binder. In the alternative, the pigment image remaining on the injecting electrode may be transferred to another surface and fixed thereon. As explained in greater detail below, this system can produce either monochromatic or polychromatic images depending upon the type and number of pigments suspended in the carrier liquid and the color of light to which this suspension is exposed in the process.

Any suitable insulating liquid may be used as the carrier for the pigment particles in the system. Typical carrier liquids are decane, dodecane, N-tetradecane, paraffin, beeswax or other thermoplastic materials, Sohio Odorless Solvent 3440 or 3454 (kerosene factions available from Standard Oil Company of Ohio) and Isopar G (a long chain saturated aliphatic hydrocarbon available from Humble Oil Company of New Jersey). Good quality images have been produced with voltages ranging from 300 to 5,000 volts in the apparatus of the FIGURE.

In a monochromatic system, particles of a single composition are dispersed in the carrier liquid and exposed to a black-and-white image. A single color results, corresponding to conventional black-and-white photography. In a polychromatic system, the particles are selected so that those of different colors respond to different wavelengths in the visible spectrum corresponding to their principal absorption bands. Also, the pigments should be selected so that their spectral response curves do not have substantial overlap, thus allowing for color separation and subtractive multicolor image formation. In a typical multicolor system, the particle dispersion should include cyan colored particles sensitive mainly to red light, magenta particles sensitive mainly to green light, and yellow colored particles sensitive mainly to blue light. When mixed together in a carrier liquid, these particles produce a black appearing liquid. When one or more of the particles are caused to migrate from base electrode 1 toward an upper electrode, they leave behind particles which produce a color equivalent to the color of the impinging light. Thus, for example, red light exposure causes the cyan-colored pigment to migrate, leaving behind the magenta and yellow pigments which combine to produce red in the final image. In the same manner, blue and green colors are reproduced by removal of yellow and magenta, respectively. When white light impinges upon the mix, all pigments migrate, leaving behind the color of the white or transparent substrate. No exposure leaves behind all pigments which combine to produce a black image. This is an ideal technique of subtractive color imaging in that the particles are not only each composed of a single component, but in addition, they perform the dual functions of final image colorant and photosensitive medium. For best results, the particle size of the pigments should be less than about 10 microns, preferably within the range of about 0.0l to 5 microns.

6 Any suitable different-colored photosensitive pig- Com und in accordance with Formula I may be ment particles having the desired spectral responses prepared b den ing 3-pyrer1e carboxaldehyde with may be used with the pigments of this invention to form ketones f h f l a partial suspension in a carrier liquid for color imaging. Typical cyan and magenta pigments include those 5 s described, for example, in U.S. Pat. Nos. 3,383,993 and 3,384,566. The magenta particles may comprise watch- R CH C R ing Red B, the barium salt of l-(methyl--chloro-azol 2 benzcne-2'-sulfonic acid)-2-hydroxy-3-napthoic acid, Cl. No. 15865 available from E, l. duPont de Nemours where R and R are as previously designated. Examand Company; the cyan particles may comprise a metples of suitable ketones within the scope of the above al-free about percent pigment by weight has been formula are acetophenone, dimethyl ketone, methyl found to produce good results. The addition of small ethyl ketone, methyl isobutyl ketone and the like. For amounts (generally ranging from 0.5 to 5 mol percent) example, 3-pyrylideneacetone corresponding to Forof electron donors or acceptors to the suspensions may 15 mula I may be prepared by a Claisen reaction as folimpart significant increases in system photosensitivity. lows:

NaOH ll [z] CHO CH CCH [Z] CH=CHCCH H 0 10 percent As previously indicated, compounds of the above By carrying the above reaction further using a molar general formulae may be prepared by conventional excess of pyrene-B-carboxaldehyde, bis pyrenyl substicondensation reactions involving the condensed polytuted ketones also corresponding to Formula I may be cyclic aromatic hydrocarbon aldehyde and an amine or obtained:

0 n n [Z]CH=CHCCH [z]-cao [Z]-CH=C1HCCH=CH [z] H O 10 percent ketone containing active hydrogen. The preferred alde- Compounds corresponding to Formula II may be prehydes are the monosubstituted carboxaldehydes of pared by catalytic reduction or hydrogenation of the condensed polycyclic aromatic hydrocarbons containalpha, beta, unsaturated ketones corresponding to Foring from about 14 to about 20 carbon atoms such as pymula I by techniques well known in the art:

R O R 0 l I l 2 ii [Z]--CH=C C- R [Z]CH- CH- C- R rene, anthracene, phenanthrene, chrysene, tetracene Hydrazones corresponding to Formula "I may be and tetraphene. These hydrocarbons may also contain prepared by an amine condensation reaction of pyreneorganic or inorganic substituent groups which do not 3-carboxaldehydewith pheny'lhydrazine:

interfere with the progress of the reaction. For exam- Z HO H NNHC H Z ple, derivatives of pyrene may be prepared by reacting l 1 C 2 6 5 a 1 CHNNHC H H O pyrene carboxaldehyde in Aldo] type condensation with the appropriate ketone having active hydrogen at (b) [Z] CHO+H NNHC H (NO [Z] the a carbon. The pyrene carboxaldehyde employed is CH=NNHC H (NO H O preferably 1, 3, 6 or 8 monosubstituted pyrene carboxaldehyde, although pyrene-3 carboxaldehyde is In the case of (b), 2,4-dm1trophenyl hydrazine is used as a reactant.

ferred.

The following reactions serve only to illustrate some Semicarbazones corresponding to Formula IV may of the methods which may be used to prepare various be prepared by the amine condensation of pyrene-3- derivative of pyrene-3-carboxaldehyde. The symbol carboxaldehyde and semicarbazide:

[ CH0 represents! Similarly, derivatives of other aromatic hydrocarbons such as anthracene, phenanthrene, chrysene, tetracene and tetraphene can be prepared in like manner using the appropriate aromatic hydrocarbon carboxaldehyde as a starting material. From the above, their structure and method of preparation are apparent to those skilled in the art.

7 DESCRIPTION OF PREFERRED EMBODIMENTS The following examples are carried out in an apparatus of the general type illustrated in the FIGURE with the imaging mix 4 coated on a NESA glass substrate through which exposure is made. The NESA glass surface is connected in series with a switch, a potential source, and the conductive center of a roller having a coating of Baryta paper on its surface. The roller is approximately 2% inches in diameter and is moved across the plate surface at about 1.45 centimeters per'second.

The plate employed is roughly 3 inches square and is exposed with a light intensity of 8,000 foot candles as measured on the uncoated NESA glass surface. Unless otherwise indicated, about 13.5 percent by weight of the indicated pigments in each example are suspended in Sohio Odorless Solvent 3454 and the magnitude of the applied potential is 2,500 volts. All pigments which have a relatively large particle size as made are ground in a ball mill for 48 hours to reduce their size to provide a more stable dispersion which improves the resolution of the final images. The exposure is made with a 3,200K lamp through a positive transparency.

EXAMPLE I About 13.5 parts of a pigment having the formula:

prepared by reacting pyrene-3-carboxaldehyde with methyl ethyl ketone is suspended in about 100 parts Sohio Odorless Solvent 3454, a kerosene fraction available from Standard Oil Company of Ohio. The suspension is coated onto the NESA glass and a positive potential is imposed on the roller electrode. The plate is exposed to an image through a positive black-andwhite transparency. A good image in yellow on the white or transparent background, conforming to the original, was produced on the negative NESA. The procedure was repeated with a negative potential imposed on the roller electrode. An image in yellow was produced on the positive NESA.

EXAMPLE II About 13.5 parts of a pigment having the formula:

is suspended in about 100 parts of solvent and the images developed as in Example I. A good orange image was produced on the negative NESA with a slight image on the positive NESA.

EXAMPLE III About 13.5 parts of a pigment having the formula:

is suspended'in parts of solvent and the images developed as in Example I. A good orange image was produced on both the negative and the positive NESA.-'

EXAMPLE 1v About 13.5 parts of a pigment having the formula:

c11 cn ccn CH2 is suspended in 100 parts of solvent and the images developed as in Example I. A good yellow image was produced on the negative NESA, with a slight image on the positive NESA.

EXAMPLE V About 13.5 parts of a pigment having the formula:

is suspended in 100 parts of solvent and the images de veloped as in Example I. A good yellow-orange image was produced on both the negative and positive NESA.

EXAMPLES VI -VIII In the same manner as detailed in Example I, pigments having the following formulae are suspended in solvent and imaged. All produce a good yellow or yellow-orange image on the negative and positive NESA.

Example VI Example VII 1n the following example, a suspension comprising three colored pigments is made by dispersing the pigments in finely divided form in Sohio Odorless Solvent 3440 so that the pigments constitute 8 percent by weight of the mixture. The mixture may be referred to as a trimix. The trimixes are individually tested by coating them on the NESA glass plate and exposing them as in Example 1 above, except that a multicolor transparency is used in place of the black-and-white transparency. Thus, a multicolored image is projected on the plate as the roller electrode moves across the surface thereof. A Baryta paper blocking electrode is employed and the roller is held at a positive potential of about 2,500 volts. The roller is passed over the plate, after which the quality of the image upon the plate is evaluated as to the image density and color purity.

EXAMPLE IX A trimix ink was prepared using 1 part yellow compound of Example VI, 1 part Monolite Fast Blue GS, and 2 parts watching Red B. CI No. 15865. When exposed, as discussed above, the trimix produced a full color image on the negative NESA corresponding to the original with good color separation and excellent density characteristics.

As shown by the above examples, the class of pig ments of the present invention having the above general formula are suitable for use in electrophoretic imaging processes. Sinch their photographic speed, density characteristics and color characteristics vary, a

mixture of the particular pigments may be preferred for' specific uses. Some characteristics of the pigments may be improved by particular purification processes, recrystallization processes and dye sensitization.

The compositions of the general formula given above are also useful in xerographic imaging systems. For use in such processes, xerographic plates may be produced by coating a relatively conductive substrate, e.g., aluminum or paper, with a dispersion of particles of the photosensitive pigment of the above general formula in a resin binder. The pigment-resin layer may also be cast as a self-supporting film. The plate formed may be both with or without an overcoating on the photoconductive layer. As a third alternative to the above noted self-supporting layer and substrate supported layer, the photosensitive pigment-resin photoconductive layer may be used in the formation of multilayer sandwich configurations adjacent a dielectric layer, similar to that shown by Golovin, et al., in the publication entitled A New Electrophotographic Process, Effected by Means at Combined Electret Layers, Doklady Akad. NaukSSR vol. 129, No. 5, pp. 1008-1011, November-December 1959.

When it is desired to coat the pigmented resin film on a substrate, various supporting materials may be used. Suitable materials for this purpose include aluminum,

steel, brass, metalized or tin oxide coated glass, semiconductive plastics and resins, paper and other convenient materials. Any suitable dielectric material may be used to overcoat the photoconductive layer. A typical overcoating is bichromated shellac.

Any suitable organic binder or resin may be used in combination with the pigment to prepare the photoconductive layer of this invention. In order to be useful the resin used in the present invention should be more resistive than about 10 and preferably more than 10 ohms per centimeter under the conditions of xerographic use. Typical resins include thermoplastics such as polyvinyl chloride, polyvinylacetates, polyvinylidene chloride, polystyrene, polybutadiene, polymethacrylates, polyacrylics, polyacrilonitrile, silicone resins, chlorinated rubber, and mixtures and copolymers thereof where applicable; and thermosetting resins such as epoxy resins including halogenated epoxy and phenoxy resins, phenolics, epoxyphenolic copolymers, epoxy ureaformaldehyde copolymers, epoxy melamine formaldehyde copolymers and mixtures thereof where applicable. Other typical resins are epoxy esters, vinyl epoxy resins, tall-oil modified epoxies, and mixtures thereof where applicable. In addition to the above noted binder materials, any other suitable resin may be used if desired. Also, other binders such as paraffin and mineral waxes may be used if desired.

The pigments may be incorporated in the dissolved or melted binder resin by way suitable means such as strong shear agitation, preferably with simultaneous grinding. These include ball milling, roller milling, sand milling, ultrasonic agitation, high-speed blending and any desirable combination of these methods. Any suitable range of pigment-resin ratios may be used.

The pigment-resin-solvent slurry (or the pigmentresin melt) may be applied to the conductive substrate by any of the well known painting or coating methods, including spraying, flow coating, knife coating, electrocoating, Mayer bar drawdown, dip coating, reverse foil coating, etc. Spraying in an electric field may be preferred for the smoothest finish and dip coating for convenience in the laboratory. The setting, drying and/or curing steps for these plates are generally similar to those recommended for films of the particular binder used for other painting applications. For example, pigment-epoxy plates may be cured by adding a cross-linking agent and stoving according to approximately the same schedule as other baking enamels made with the same resins and similar pigments for painting applications. A very desirable aspect of these pigments is that they are stable against chemical decomposition at the temperatures normally used for a wide variety of bakeon enamels, and therefore, may be incorporated in very hard glossy photoconductive coatings, similar to automotive or kitchen appliance resin finishes.

The thickness of the photoconductive films may be varied from about 1 to about 100 microns, depending on their required individual purpose. Self-supporting films, for example, cannot usually be manufactured in thicknesses thinner than about 10 microns, and they are easiest to handle and use in the 15 to micron range. Coatings, on the other hand, are preferably formed in the 5 to 30 micron range. For certain compositions and purposes, it is desirable to provide an overcoating; this should usually not exceed the thickness of the photoconductive coating, and preferably not above one-quarter of the latter. Any suitable overcoating material may be used, such as bichromated shellac.

The invention as it pertains to xerographic imaging processes will be further described with reference to the following examples, which describes in detail a preferred embodiment of the present invention. Parts, ratios and percentages are by weight unless otherwise stated.

A xerographic plate for use as in the following example is prepared as follows: mixtures using specific pigments and resin binders are prepared by ball milling the pigment and a solution of a resinous binder and one or more solvents until the pigment is well dispersed. This is done by adding the desired parts of the pigment to the desired parts of resin solution in a suitable mixing vessel. A quantity of /8 inch steel balls are added and the vessel is rotated for approximately one-half hour in order to obtain a homogeneous dispersion. The cooled slurry is applied onto an aluminum substrate with a wire drawdown rod and force dried in an oven for about 3 minutes at about 100C. The coated sheets are dark rested for about 1 hour and then tested.

In the following example the plate is tested as follows. The plate is charged negative by corona discharge to about 400 volts and exposed to a light and shadow image. The plate is cascade developed using Xerox 1824 developer. The powder image produced on the plate corresponds to the projected image. The developed image may be then either fused to the plate or may be electrostatically transferred to a receiving sheet and there fused. Where the image is transferred, the plate may be then cleaned or residual toner and may be reused as by the above described process.

EXAMPLE X The xerographic plate is prepared by initially mixing about 2 parts of Lucite 2042, an ethylmethacrylate polymer, about 18 parts benzene, and about 1 part of the pigment of Example VII. This mixture is coated onto an aluminum substrate to a thickness of about 8 microns and cured. The plate is then charged, exposed for about 45 seconds to a'light and shadow image using a Simmons Omega D3 enlarger equipped with a tungsten light source operating at 2950K color temperature (illumination level incident on the palte is 2.8 foot candles as measured with a Weston Illumination Meter Model No. 756), and developed as above described. The image produced is heat fused directly onto the plate. The image produced was found to be satisfactory.

Although specific components and proportions have been described in the above examples, other suitable materials, as listed above, may be used with similar results. In addition, other materials may be added to the pigment compositions to synergize, enhance, or otherwise modify their properties. The pigment compositions of this invention may be dye sensitized, if desired, or may be mixed with other photosensitive materials, both organic and inorganic.

Other modifications and ramifications of the present invention will occur to those skilled in the art upon a reading of the present disclosure. These are intended to be included within the scope of this invention.

What we claim is:

1. An electrophoretic imaging suspension comprising a electrically insulating liquid carrier medium having dispersed therein an electrically photosensitive pigment composition comprising a mixture of pigments, at least a first of said pigments selected from the group consisting of cyan, magneta, or mixtures thereof. and

another of said pigments comprising a pigment having the formula:

[R l HEY wherein:

represents a l, 3, 6 or 8 monosubstitued pyrene moiety.

3. The suspension of claim 1 wherein represents 4. The suspension of claim 3 wherein the pigment comprises a compound having the formula.

. cH=NNH-@ 5. The suspension of claim 3 wherein the pigment comprises a compound having the formula:

7. The suspension of claim 6 wherein said substantially insulating carrier medium comprises a liquid aliphatic hydrocarbon.

13 14 N02 6. The suspension of claim 1 wherein said first pigment comprises a mixture of cyan and magenta pig- CH=NNH N02 ment5 

1. AN ELECTROPHORETIC IMAGING SUSPENSION COMPRISING A ELECTRICALLY INSULATING LIQUID CARRIER MEDIUM HAVING DISPERSED THEREIN AN ELECTRICALLY PHOTOSENSITIVE PIGMENT COMPOSITION COMPRISING A MIXTURE OF PIGMENTS, AT LEAST A FIRST OF SAID PIGMENT SELECTED FROM THE GROUP CONSISTING OF CYAN, MAGNETA, OR MIXTURES THEREOF, AND ANOTHER OF SAID PIGMENTS COMPRISING A PIGMENT HAVING THE FORMULA0: ($-CH=N-NH-),(R3)N-BENZENE WHEREIN: $ REPRESENTS A CONDENSED POLYCYCLIC AROMATIC GROUP HAVING FROM ABOUT 14 TO ABOUT 20 CARBON ATOMS: R3 IS SELECTED ROM THE GROUP CONSISTING OF: H, OH, CH3, OCH3, C2H5, OC2H5, COCH3, COCH3, COC2H5, CO2C2H5, NO2, CN, SO2NH2, SO2NHC6H5, CL, F. BR, 1, AND N IS A POSITIVE INTEGER FROM 1-3.
 2. The suspension of claim 1 wherein
 3. The suspension of claim 1 wherein
 4. The suspension of Claim 3 wherein the pigment comprises a compound having the formual:
 5. The suspension of claim 3 wherein the pigment comprises a compound having the formula:
 6. The suspension of claim 1 wherein said first pigment comprises a mixture of cyan and magenta pigments.
 7. The suspension of claim 6 wherein said substantially insulating carrier medium comprises a liquid aliphatic hydrocarbon. 